To explore the possibility that the HLA-B27 antigen may exist in more than one form, murine monoclonal antibodies were produced to B27 molecules. The first such antibody, anti-B27M1, reacts with 100% of B27 + cells and, therefore, does not distinguish B27 + healthy individuals from those with spondyloarthropathies. A second antibody, anti-B27M2, recognizes a variant of the B27 molecule that is present in most but not all Caucasian B27 + individuals. The frequency of B27M2(+) variants also appears to differ among different ethnic groups. Preliminary studies suggest that B27 + patients with ankylosing spondylitis are less likely than B27 + healthy individuals to express the B27M2 variant.